The present invention relates to an automatic duplex electrophotographic copying machine having a duplex copying-related transport passage under a single side copying-related transport passage and an intermediate tray provided in the way of the duplex copying-related transport passage, in which a copy paper having a transferred image on one side is temporarily stored in the intermediate tray before being fed to a copying process section.
To meet the recent demand for simplified copying operation and multiple functions of a copying machine, various types of copying machines with automatic duplex copying function have been introduced to the market. In such an automatic duplex copying machine, a duplex copying-related transport passage is installed under a single side copying-related transport passage, communicating from a copy paper discharge outlet to the upper stream of the copying process section. An intermediate tray and switch-back means for reversing the transport direction of a copy paper are provided in the way of the duplex copying-related transport passage. Means for selecting transport direction of a copy paper is also provided in the vicinity of the copy paper discharge outlet. In the single side copying mode, a copy paper having a transferred image on one side is transported to be discharged outside the copying machine. In the duplex copying mode, the copy paper is transmitted to the duplex copying-related transport passage and temporarily stored in the intermediate tray. When the original document on the document rest is replaced with a new original document, the copy paper having a transferred image on one side is transmitted from the intermediate tray to the copying process section where the new original document is copied on the other side of the copy paper.
In operating the conventional automatic duplex electrophotographic copying machine of the above mentioned construction, two or more copy papers may be supplied simultaneously in a layer to the copying process section when the first original document is to be copied. If this occurs, a blank copy paper on which no image is transferred on neither side enters the intermediate tray. When the duplex copying process starts, the blank copy paper is supplied to the copying process section. As a result, copy papers discharged after the duplex copying process include a defective copy paper that has no image transferred on the first side. In general, the operator cannot be informed even if two or more copy papers are supplied to the copying process section for copying on the first side. Besides, the conventional automatic duplex copying machine is not equipped with means for separating blank copy papers from the single sided copy papers supplied from the intermediate tray. Therefore, it is unavoidable for the operator to continue duplex copying operation without noticing the defective copy paper. As a result, it is necessary to repeat duplex copying operation when the copy paper is found to be defective after it has been discharged from the paper outlet. This disadvantage significantly impairs the operation efficiency of the copying machine.